LibWeb+LibXML: Cache the result of parsing the XHTML DTD and reuse it

There's no reason to parse this again and again every time we parse an
XHTML document.

Userland/Libraries/LibWeb/XML/XMLDocumentBuilder.h

@@ -21,7 +21,7 @@ enum class XMLScriptingSupport {
     Enabled,
 };
 
-ErrorOr<ByteString> resolve_xml_resource(XML::SystemID const&, Optional<XML::PublicID> const&);
+ErrorOr<Variant<ByteString, Vector<XML::MarkupDeclaration>>> resolve_xml_resource(XML::SystemID const&, Optional<XML::PublicID> const&);
 
 class XMLDocumentBuilder final : public XML::Listener {
 public:

Userland/Libraries/LibXML/Parser/Parser.cpp

@@ -573,15 +573,21 @@ ErrorOr<void, ParseError> Parser::parse_doctype_decl()
             if (m_options.resolve_external_resource) {
                 auto resource_result = m_options.resolve_external_resource(doctype.external_id->system_id, doctype.external_id->public_id);
                 if (!resource_result.is_error()) {
-                    StringView resolved_source = resource_result.value();
-                    TemporaryChange source { m_source, resolved_source };
-                    TemporaryChange lexer { m_lexer, LineTrackingLexer(m_source) };
-                    auto declarations = TRY(parse_external_subset());
-                    if (!m_lexer.is_eof()) {
-                        return parse_error(
-                            m_lexer.current_position(),
-                            ByteString::formatted("Failed to resolve external subset '{}': garbage after declarations", doctype.external_id->system_id.system_literal));
-                    }
+                    auto declarations = TRY(resource_result.release_value().visit(
+                        [&](ByteString resolved_source) -> ErrorOr<Vector<MarkupDeclaration>, ParseError> {
+                            TemporaryChange source { m_source, resolved_source.view() };
+                            TemporaryChange lexer { m_lexer, LineTrackingLexer(m_source) };
+                            auto declarations = TRY(parse_external_subset());
+                            if (!m_lexer.is_eof()) {
+                                return parse_error(
+                                    m_lexer.current_position(),
+                                    ByteString::formatted("Failed to resolve external subset '{}': garbage after declarations", doctype.external_id->system_id.system_literal));
+                            }
+                            return declarations;
+                        },
+                        [&](Vector<MarkupDeclaration> declarations) -> ErrorOr<Vector<MarkupDeclaration>, ParseError> {
+                            return declarations;
+                        }));
                     doctype.markup_declarations.extend(move(declarations));
                 }
             }
@@ -1755,7 +1761,10 @@ ErrorOr<ByteString, ParseError> Parser::resolve_reference(EntityReference const&
                     if (result.is_error())
                         return parse_error(m_lexer.position_for(0), ByteString::formatted("Failed to resolve external entity '{}': {}", reference.name, result.error()));
 
-                    resolved = result.release_value();
+                    if (!result.value().has<ByteString>())
+                        return parse_error(m_lexer.position_for(0), ByteString::formatted("Failed to resolve external entity '{}': Resource is of the wrong type", reference.name));
+
+                    resolved = result.release_value().get<ByteString>();
                     return {};
                 }));
             break;

Userland/Libraries/LibXML/Parser/Parser.h

@@ -49,7 +49,7 @@ public:
         bool preserve_cdata { true };
         bool preserve_comments { false };
         bool treat_errors_as_fatal { true };
-        Function<ErrorOr<ByteString>(SystemID const&, Optional<PublicID> const&)> resolve_external_resource {};
+        Function<ErrorOr<Variant<ByteString, Vector<MarkupDeclaration>>>(SystemID const&, Optional<PublicID> const&)> resolve_external_resource {};
     };
 
     Parser(StringView source, Options options)
@@ -70,6 +70,8 @@ public:
 
     Vector<ParseError> const& parse_error_causes() const { return m_parse_errors; }
 
+    ErrorOr<Vector<MarkupDeclaration>, ParseError> parse_external_subset();
+
 private:
     struct EntityReference {
         Name name;
@@ -138,7 +140,6 @@ private:
     ErrorOr<StringView, ParseError> parse_system_id_literal();
     ErrorOr<StringView, ParseError> parse_cdata_section();
     ErrorOr<ByteString, ParseError> parse_attribute_value_inner(StringView disallow);
-    ErrorOr<Vector<MarkupDeclaration>, ParseError> parse_external_subset();
     ErrorOr<void, ParseError> parse_text_declaration();
 
     ErrorOr<void, ParseError> expect(StringView);

Userland/Utilities/xml.cpp

@@ -362,7 +362,7 @@ static auto parse(StringView contents)
         contents,
         {
             .preserve_comments = true,
-            .resolve_external_resource = [&](XML::SystemID const& system_id, Optional<XML::PublicID> const&) -> ErrorOr<ByteString> {
+            .resolve_external_resource = [&](XML::SystemID const& system_id, Optional<XML::PublicID> const&) -> ErrorOr<Variant<ByteString, Vector<XML::MarkupDeclaration>>> {
                 auto base = URL::create_with_file_scheme(s_path);
                 auto url = URL::Parser::basic_parse(system_id.system_literal, base);
                 if (!url.is_valid())